1. Field of the Invention
The present invention generally relates to systems for reading and writing binary information onto magneto-optical memory disks and, more particularly, to such systems that employ modulated magnetic heads for direct overwriting.
2. State of the Art
Magneto-optical memory disks each contain a very large number of discrete magnetic sites, or "domains," that are arranged almost side-by-side in concentric or spiral data tracks on a common recording surface or "data plane." Although a data plane on a magneto-optical disk is normally parallel to the face of the disk, it is not necessary that a data plane lay on the disk's surface. A transparent magneto-optical disk, for example, can have a data plane that is arranged between its faces. Typically, a data plane at a disk's surface is covered by a thin protective coating.
When a focused beam of linearly polarized light is reflected from a magnetic domain on the data plane of a magneto-optical memory disk, the polarity of the domain (i.e., its magnetic moment) can cause the electric field vector of the reflected light to be rotated. By making use of this phenomenon, information can be stored on a magneto-optical disk by purposefully magnetizing patterns of domains along tracks on the disk's recording surface. Then, the polarization of the light reflected from the disk's recording surface can be coded to retrieve the stored binary information when reading the disk.
In the magneto-optical memory disk recording art, the process of purposefully magnetizing domains along tracks on a recording surface is referred to as "writing". To write information onto a magneto-optical memory disk, it is necessary to maintain the point of focus of a laser beam coincident with the disk's recording surface as the disk rotates. The laser beam functions to heat individual areas along the recording tracks to a temperature sufficient to lower their coercivity to a point where their magnetic moment can be easily altered by a magnetic recording head, thereby forming magnetic domains. The recorded magnetic moments remain after the domains cool.
Because of the minute size of magnetic domains on magneto-optical memory disks, it is critical to precisely maintain the position of the point of focus of a laser beam on the data plane through an objective lens when writing onto, and reading from a magneto-optical disk. Conventionally, systems for maintaining the point of focus of a laser beam on the recording surface of a magneto-optical disk include an objective lens mounted on a voice-coil motor. In operation, the voice coil motor is controlled to selectively move the lens toward and away from the disk surface as required to compensate for changes in the axial position of the data plane of a disk.
Axial-direction disturbances to the point of focus of an objective lens relative to the data plane of an optical memory disk can be caused by various factors. For example, axial-direction disturbances can be caused by temperature changes, by vibrations, by mechanical shocks and by other environmental factors. In addition, the location of the point of focus of an objective lens relative to the data plane of an optical memory disk can be disturbed by axial-direction variations in disc geometry due, for instance, to factors such as disk warpage, droop, waviness, and thickness variations.
As mentioned above, writing of binary information onto a magneto-optical memory disk requires the use of a magnetic head. In conventional writing operations for magneto-optical memory disks, a laser is modulated (i.e., switched) so that its light heats only areas on a rotating magneto-optical disk that have been previously magnetically erased. As the disk is rotated during the writing operation, the heated areas are exposed to a magnetic writing field whose orientation is opposite to the orientation of the erased areas. As a result, selected magnetic domains are produced, or written, such that the direction of their magnetic moment corresponds to the direction of the magnetic writing field.
One known alternative to the above-described technique for writing binary information onto magneto-optical disks is the so-called "direct overwriting" technique. Direct overwriting has the benefit that it can be applied to areas of a magneto-optical disk that have not been previously erased magnetically, thus reducing the time required to write to a disk by eliminating the erasing operation that normally is required before writing. In conventional practice, direct overwriting systems for magneto-optical memory disks include a laser source which is mounted on one side of a disk in precise alignment with a magnetic head which is mounted on the opposite side of the disk. More particularly, alignment between the magnetic head and the laser source must be such that the magnetic field and the point of focus of the laser light are coincident on the data plane of the disk.